1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor chips for display, and more particularly to a method of thermally treating a semiconductor thin film with laser irradiation.
2. Description of Related Art
Liquid crystal display devices which are designed in a large size and with high minuteness and in which polycrystal silicon thin film transistors are used as switching elements have been developed with much hope. In order to mass-produce liquid crystal displays with polycrystal silicon thin film transistors in a large size and with high minuteness, it is indispensable to establish a low-temperature process in which low-price glass substrates are usable. A technique that a laser beam is irradiated onto a semiconductor thin film of amorphous silicon or the like to form high-quality polycrystal silicon on a glass substrate having a low melting point has been greatly expected as a method to perform the low-temperature process.
FIG. 1 is a schematic diagram showing a previous paper suggested laser beam irradiation method. A semiconductor chip 101 for display which is a target to be processed has such a laminating structure that a semiconductor thin film 103 is formed on a transparent insulating substrate 102. In this method, a laser beam 105 is irradiated onto a predetermined sectioned area 104 which is provided on the semiconductor thin film 103. In the conventional method, the output power of the laser beam is limited to a small level, and thus the maximum area which can be irradiated with one-shot laser irradiation of laser is limited to a narrow area about 100 .mu.m.sup.2. Accordingly, when a semiconductor thin film 103 having a large area is required to be processed to satisfy a requirement for a large-scale picture, the laser beam is irradiated onto the whole semiconductor thin film while scanning the laser beam 105 or shifting the laser-irradiation area stepwisely. That is, it has been hitherto considered important to increase the energy density of the laser beam rather than to narrow the laser-irradiation area down. With increase of the energy density, a semiconductor thin film of amorphous silicon or polycrystal silicon having relatively small grain size is perfectly melted to increase its grain size. In this method, however, an irradiation time per chip increases and thus manufacturing throughput is reduced. Furthermore, the scanning of the irradiation of the laser beam causes temperature difference to occur locally, and thus causes increase in variability of crystal grain size. Therefore, wide variations occur in electrical characteristics of the thin film transistors such as mobility, a threshold voltage, etc.
The above point will be described in detail with reference to FIG. 2.
In the method as described above, the semiconductor thin film 103 having a large area is crystallized by irradiating a laser beam onto a small area while scanning the laser beam as shown in FIG. 2. Accordingly, non-uniformity of crystallization occurs at an overlap area 106 between a laser shot and a next laser shot, so that the electrical characteristics of thin film transistors formed at the overlap area 106 are uneven. For example, the overlap area 106 is subjected to the laser irradiation several times, whereas the other areas are subjected to the laser irradiation only once, so that the heating temperature for the semiconductor thin film is also locally uneven.
In addition to the method as described above, various laser irradiation systems have been hitherto proposed. For example, in a method of manufacturing a semiconductor device as disclosed in Japanese Laid-open Patent Application No. Sho-60-245124, a laser pulse having wavelength of 150 nm to 350 nm is irradiated at an energy density of 200 to 500 mJ/cm.sup.2 to promote crystallization of a semiconductor thin film. In this system, an amorphous area and a crystal area coexist on a substrate, and thin film transistors are integrated over the two areas. Accordingly, the electrical characteristics of the thin film transistors vary between both the amorphous area and the crystal area, and thus controllability is lost.
Furthermore, in a method of manufacturing a semiconductor memory as disclosed in Japanese Laid-open Patent Application No. Hei-3-273621, a laser annealing treatment is performed on a memory-cell basis (microarea in several tens .mu.m order), and non-irradiated areas remain in between memory cells. Therefore, it is impossible to irradiate a laser beam onto a large-scale circuit at the same time.
Still furthermore, in a method of manufacturing a liquid crystal display device as disclosed in Japanese Laid-open Patent Application No. Hei-5-66422, for crystallization of a semiconductor thin film, a one (single) shot of laser pulse is irradiated onto each of areas on which a horizontal scanning circuit and a vertical scanning circuit respectively will be formed. In this case, it is necessary to make crystallized areas continuous, and thus the crystal particle size is dispersed at a linking boundary between the laser-irradiated areas.